This invention pertains to a method and apparatus for estimating the amount of evaporative fuel emissions from a vehicle during a test period.
Light duty automotive vehicles sold in the United States since the 1996 model year have been required to incorporate an on-board self-diagnostic system that satisfies the U.S. federal OBDII requirements. The self-diagnostic systems comprise sensors and a microprocessor to monitor various vehicle systems including its exhaust and fuel evaporative emission control systems. As previously disclosed in U.S. Pat. Nos. 5,431,042 and 5,750,886, it is possible to monitor an automotive vehicle""s operation while the engine is on by using standard diagnostic messages to periodically poll the engine controller.
In accordance with the disclosures of the ""042 and ""886 patents, a microprocessor module that is connected to the diagnostic link and to the engine or powertrain controller sends and receives messages that are used in the practice of those inventions. Running totals are kept in the module""s non-volatile memory of values that can be used to estimate the vehicle""s cumulative exhaust emissions of carbon monoxide, hydrocarbons and nitrogen oxides. For example, specific quantities monitored by the module in accordance with these patents include the number of engine starts binned by the temperature of the engine coolant at the time of the start, the total time of engine operation, the total time the engine is operated in driver commanded enrichment mode, and the distance traveled with the vehicle malfunction indicator light (MIL) on for various OBDII classes of malfunctions.
The functional relationship between the cumulative totals collected in the exhaust analyzer module and the cumulative emission of a given pollutant from a particular vehicle can be based on mathematical models that have been developed to estimate, for example, the total emissions in an air basin from all the vehicles operating in it. One such model is EMFAC that was developed by the California Air Resources Board. Another model is Mobile 6 that is being developed by the U.S. Environmental Protection Agency. Thus, data accumulated by the practices of the ""042 and ""886 patents may, for example, be stored on-board in the described micro-processor module and downloaded by a technician for use in such a model to estimate the cumulative exhaust emissions of the vehicle over a test period. Another use of the patent practices is to apprise the driver of the effects of his operation of the vehicle on such emissions.
Evaporative emissions from individual vehicles are also an important component of the hydrocarbon emissions inventory from a motor vehicle fleet. Hydrocarbon emissions are regulated because they are a precursor to ozone formation. The chemical reaction that converts hydrocarbons and nitrogen oxides into ozone is promoted by high ambient temperature. High ambient temperature also increases the evaporative emissions of hydrocarbons. On some days that the Los Angeles air basin has exceeded the allowed concentration of ozone, it has been estimated that more than half of the hydrocarbon inventory resulted from fuel evaporated from vehicles. Much of the evaporative emissions come from older vehicles without evaporative controls or from vehicles with leaks or malfunctioning controls.
Gasoline engine-powered vehicles are susceptible to evaporative fuel loss because of the volatility of the fuel. The temperature in the fuel tank increases due to ambient heating of the fuel, or to hot fuel returned from the engine compartment, and can cause the liquid fuel to vaporize. Current gasoline tanks are vented through a tube that conducts evaporated fuel to a carbon particle-filled canister in the engine compartment of the vehicle. Gasoline hydrocarbons are temporarily adsorbed on the carbon particles to reduce or eliminate release of the hydrocarbons to the atmosphere. At suitable times during engine operation, the engine controller signals the opening of a canister purge valve to engine vacuum. Ambient air is thus permitted to flow through the canister, removing stored hydrocarbons and carrying them into the engine where they are burned. The complete avoidance of release of evaporated fuel to the atmosphere depends upon such purging of stored fuel from the canister before it is overloaded and discharges fuel to the atmosphere and upon the detection and closing of other leaks in the evaporative emission control system.
Evaporative emissions from vehicles are thus attributed to the following general categories. Diurnal emissions are those occurring when the engine is not running and driven by the daily cycle of ambient temperature increase and decrease. If the diurnal cycle is interrupted by engine start-up, a partial diurnal loss period may have to be considered. The analysis of emission losses thus contemplates an engine-off resting period which is the baseline measured during the test for diurnal emissions. The quantity of resting emissions is sometimes modeled as a function of the lowest temperature occurring during the resting period. There is also a hot soak category that includes emissions that occur shortly after the engine is turned off. The running loss category includes those evaporative emissions that occur while the engine is running and during the hot soak period. There are also leaks of liquid fuel.
Practices disclosed in the above patents can be used to estimate running loss and hot soak emissions because they are related to engine-on data. However, there remains a need for methods and apparatus for collecting suitable information during periods when the engine is not running to estimate evaporative emissions attributable to diurnal and partial diurnal losses as well as losses attributable to malfunctions of the evaporative emission control system. Such information would be used with a module like EMFAC and Mobile 6 to determine evaporative emissions during a test period.
The invention provides a method of determining cumulative evaporative emissions of hydrocarbons from an automotive vehicle during a test period, suitably when the engine is not running. The vehicle has an engine that is operated under a microprocessor-based engine controller and a fuel evaporation emission control system comprising a fuel tank for hydrocarbon fuel and fuel vapor adsorption means connected to said tank and said engine. The adsorption means is usually a canister of carbon particles used to temporarily store evaporated hydrocarbons flowing from the fuel tank. When the engine runs, engine vacuum promotes air flow through the canister into the engine to strip hydrocarbons from the canister and carry them into the combustion cylinders of the engine where they are burned. Preferably, the vehicle also has a self-diagnostic system (e.g., as required under OBDII) that detects malfunctions in said evaporation emission control system.
An evaporative emissions microprocessor based module is provided including suitable memory and input-output components and a temperature sensor to practice the process aspect of the invention. The module is connected with suitable data links to the engine controller, the self-diagnostic system, if present and, preferably to a download port for outside the vehicle processing of the evaporative emission data acquired during a vehicle test. The emission module including a temperature sensor is suitably located in the passenger compartment behind the vehicle instrument panel. Optionally, the temperature sensor could be located in the fuel tank.
In a preferred embodiment, the process comprises periodically interrogating said diagnostic system during engine operation for defects in said emission control system and recording said defects, if any, in a microprocessor readable memory. The module sensor is used to measure the temperature at regular, predetermined intervals of time during all occasions during a test period when the engine is not running. The temperature is recorded in a readable memory preferably after a predetermined engine off hot soak period. After a suitable period, preferably after 24 hours, the lowest recorded temperature is determined as a basis for diurnal or partial diurnal cumulative emission determinations.
Meaningful temperature data is thus accumulated during a vehicle test of desired length. Diagnostic data pertaining to the performance of the evaporative control system is obtained by the emissions module from the vehicle OBDII system, if present, during vehicle operation. Most of the temperature data is recorded in the module when the engine is not running. Preferably, any single engine-off test period is terminated after a predetermined period, such as about three days, to prevent excessive battery drain.
The cumulative evaporative emissions during said test period are then determined as a function of the cumulative effect of said malfunctions, if any, and the cumulative effect of said diurnal and partial diurnal temperatures. There are suitable mathematical evaporative emission models for the purpose of the determination. The models may be loaded into the memory of the evaporative emissions model. Preferably, however, the data from the emissions module is downloaded upon demand to an external processor for the calculation of the evaporative emissions during the test period.
Other objects and advantages of the invention will become more apparent from a detailed description of a preferred embodiment, which is provided below. Reference will be had to the drawing figures which are described in the next section.